This invention relates to a data compression method, and in particular concerns a method for transmitting data in a compressed form in real time. The invention may be used in devices such as vehicle safety systems in which the speed of transmittal of data is important.
Both when transmitting and storing data it is often desirable to compress the data to allow quick transmission thereof, to enable use of cheaper transmission lines having lower transmission capacities, or to ensure that less memory space is required to store the data. Many types of data contain redundant information which may be discarded before transmission or storage. The amount of redundant information present in a quantity of data will depend in part upon the type of the data. By using an appropriate algorithm, it is often possible to discard some or all of the redundant information to reduce the quantity of data before transmittal or storage thereof, and to recover this information when the data is to be used again.
Compression algorithms for use with a quantity of data representing a series of consecutive values of a parameter—for example, a sound signal from a microphone or an output from a volt meter or accelerometer—may need to use values which appear before and as well as after a current value in the series of values. If this is the case, there will be a delay involved in the compression of the data, as the algorithm waits for the required subsequent value to be received. In the case of a quantity of data representing an image, the delay due to compression may be larger still, because values relating to lines adjacent a current line in the image may need to be used.
For certain applications it is, however, important that the delays involved in transmission of data are minimized. An example of such circumstances is the transmission of data from a crash sensor, for instance an accelerometer, to a control unit that controls the deployment of a safety device in a vehicle. If a crash sensor has determined that the vehicle is involved in a crash situation, any delay before, for example, an airbag is inflated should clearly be as short as possible.
It is an object of the present invention to provide a method for transmitting data in a compressed form involving minimal delays.